Electric machine having a high-torque switched reluctance motor

ABSTRACT

According to one embodiment of the present invention, an electric machine comprises a stator and a rotor. The stator has at least one stator pole with a first leg and a second leg. The rotor has at least one rotor pole. The rotor rotates relate to the stator. The at least one rotor is configured to rotate between the first leg and the second leg of the at least one stator pole.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.60/672,258, filed on Apr. 18, 2005, and is a Continuation-in-Part ofU.S. patent application Ser. No. 11/369,202 filed on Mar. 6, 2006, whichis a continuation of U.S. application Ser. No. 10/359,488, filed on Feb.5, 2003 now U.S. Pat. No. 7,008,200. U.S. patent application Ser. No.11/369,202, is incorporated by reference herein.

TECHNICAL FIELD OF THE INVENTION

This invention relates in general to electric machines and, moreparticularly, to a high-torque switched reluctance motor.

BACKGROUND OF THE INVENTION

Switched reluctance motors (SRM) generally include componentsconstructed from magnetic materials such as iron, nickel, or cobalt. Apair of opposing coils in the SRM may become electronically energized.The inner magnetic material is attracted to the energized coil causingan inner assembly to rotate while producing torque. Once alignment isachieved, the pair of opposing coils is de-energized and a next pair ofopposing coils is energized.

SUMMARY OF THE INVENTION

According to one embodiment of the present invention, an electricmachine comprises a stator and a rotor. The stator has at least onestator pole with a first leg and a second leg. The rotor has at leastone rotor pole. The rotor rotates relative to the stator. The at leastone rotor is configured to rotate between the first leg and the secondleg of the at least one stator pole.

Certain embodiments of the invention may provide numerous technicaladvantages. For example, a technical advantage of one embodiment mayinclude the capability to increase the symmetry of poles in anelectrical machine to increase torque. Other technical advantages ofother embodiments may include the capability to allow very small gaps inan electrical machine to be maintained, even when components deform dueto thermal and centrifugal effects. Other technical advantages of otherembodiments may include the capability to allow external coils to beseparated from the interior of an electrical machine, which may bechemically corrosive if it is integrated with compressors, expanders, orpumps. Yet other technical advantages of other embodiments may includethe capability to utilize U-shaped poles that are electrically andmagnetically isolated from adjacent poles, thereby allowing them to bebuilt in modules for insertion into a non-magnetic frame, which may haveease of manufacture and repair. Yet other technical advantages of otherembodiments may include the capability to utilize U-shaped poles thatare external to the motor enclosure, enabling better thermal contactwith the ambient environment and reducing the tendency to overheat.Still yet other technical advantages of other embodiments may includethe capability to create a magnetic flux in a rotor of an electricalmachine that does not cross the axis of the rotor. Still yet othertechnical advantages of other embodiments may include the capability toallow the space within the interior of a rotor of an electrical machineto be available for items such as, but not limited to compressors,expanders, engines, and pumps. Although specific advantages have beenenumerated above, various embodiments may include all, some, or none ofthe enumerated advantages. Additionally, other technical advantages maybecome readily apparent to one of ordinary skill in the art after reviewof the following figures, description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

To provide a more complete understanding of the embodiments of theinvention and features and advantages thereof, reference is made to thefollowing description, taken in conjunction with the accompanyingfigures, wherein like reference numerals represent like parts, in which:

FIG. 1A shows a schematic representation of a conventional switchedreluctance motor (SRM);

FIG. 1B is a dot representation of the SRM of FIG. 1A;

FIG. 2 shows a schematic representation of a long flux path through theconventional switched reluctance motor (SRM) of FIG. 1A;

FIG. 3 shows in a chart the effect of MMF drop in the torque productionof a one-phase, one horsepower machine;

FIG. 4 shows a dot representation for a switched reluctance motor (SRM),according to an embodiment of the invention;

FIGS. 5A and 5B illustrate a rotor/stator configuration, according to anembodiment of the invention;

FIG. 6 shows an outer rotor assembly of a rotor/stator configuration,according to an embodiment of the invention;

FIG. 7 shows an inner rotor assembly of a rotor/stator configuration,according to an embodiment of the invention;

FIG. 8 shows a stator/compressor case of a rotor/stator configuration,according to an embodiment of the invention;

FIG. 9 shows a cutaway view of a composite assembly of a rotor/statorconfiguration, according to an embodiment of the invention; and

FIG. 10 shows the composite assembly of FIG. 9 without the cutaway;

FIG. 11 shows a side view of how a rotor changes shape when it expandsdue to centrifugal and thermal effects;

FIG. 12 shows a rotor/stator configuration, according to anotherembodiment of the invention;

FIGS. 13A and 13B show a rotor/stator configuration, according toanother embodiment of the invention;

FIG. 14 shows a rotor/stator configuration, according to anotherembodiment of the invention;

FIG. 15 shows an unaligned position, a midway position, and an alignedposition;

FIG. 16 shows an energy conversion loop;

FIG. 17 shows a rotor/stator configuration, according to anotherembodiment of the invention;

FIG. 18 shows a rotor/stator configuration, according to anotherembodiment of the invention;

FIG. 19 shows a rotor configuration, according to another embodiment ofthe invention;

FIG. 20 shows a rotor/stator configuration, according to anotherembodiment of the invention;

FIGS. 21A and 21B show a rotor/stator configuration, according toanother embodiment of the invention;

FIG. 22 illustrates the formation of flux lines in a SRM drive;

FIGS. 23 and 24 shows the placement of easily saturated materials orflux barriers under the surface of rotors; and

FIG. 25 shows a chart of B-H curves for various alloys.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

It should be understood at the outset that although exampleimplementations of embodiments of the invention are illustrated below,embodiments of the present invention may be implemented using any numberof techniques, whether currently known or in existence. The presentinvention should in no way be limited to the example implementations,drawings, and techniques illustrated below. Additionally, the drawingsare not necessarily drawn to scale.

Various electric machines such as motors and generators and typevariations associated with such motors and generators may avail benefitsfrom the embodiments described herein. Example type variations include,but are not limited to, switched reluctance motors (SRM), permanentmagnet AC motors, brushless DC (BLDC) motors, switched reluctancegenerators (SRG), permanent magnet AC generators, and brushless dcgenerators (BLDCG). Although particular embodiments are described withreference to one or more type variations of motor and/or generators, itshould be expressly understood that such embodiments may be utilizedwith other type variations of motors or generators. Accordingly, thedescription provided with certain embodiments described herein areintended only as illustrating examples type variations that may availbenefits of embodiments of the invention. For example, teachings of someembodiment of the invention increase the torque, power, and efficiencyof electric motors, particularly switched reluctance motors (SRM). Suchembodiments may also be used with permanent magnet AC motors andbrushless DC (BLDC) motors. Some of same advantages described withreference to these embodiments may be realized by switched reluctancegenerators (SRG), permanent magnet AC generators, and brushless dcgenerators (BLDCG).

In conventional radial and axial SRMs, the magnetic flux flows through along path through the whole body of a stator and rotor. Due to thesaturation of iron, conventional SRMs have a large drop in the magnetomotive force (MMF) because the flux path is so large. One way to reducethe loss of MMF is to design thicker stators and rotors, which reducesthe flux density. However, this approach increases the weight, cost, andsize of the machine. Accordingly, teachings of embodiment of theinvention recognize that a more desirable approach to reduce theselosses is to minimize the flux path, which is a function of geometry andtype of machine.

Teachings of some embodiments additionally introduce a new family ofstator/pole interactions. In this family, stator poles have been changedfrom a conventional cylindrical shape to U-shaped pole pairs. Thisconfiguration allows for a shorter magnetic flux path, which inparticular embodiments may improve the efficiency, torque, and powerdensity of the machine.

To take full advantage of the isolated rotor/stator structures of thisinvention, sensorless SRM and BLDC control methods may be utilized,according to particular embodiments.

The switched reluctance motor (SRM) has salient poles both on both thestator and rotor. It has concentrated windings on the stator and nowinding on the rotor. This structure is inexpensive and rugged, whichhelps SRMs to operate with high efficiency over a wide speed range.Further, its converter is fault tolerant. SRMs can operate very well inharsh environments, so they can be integrated with mechanical machines(e.g., compressors, expanders, engines, and pumps). However, due to theswitching nature of their operation, SRMs need power switches andcontrollers. The recent availability of inexpensive power semiconductorsand digital controllers has allowed SRMs to become a serious competitorto conventional electric drives.

There are several SRM configurations depending on the number and size ofthe rotor and stator poles. Also, as with conventional electricmachines, SRMs can be built as linear-, rotary-, and axial-fluxmachines. In these configurations, the flux flows 180 electrical degreesthrough the iron. Due to saturation of iron, this long path can producea large drop in MMF, which decreases torque density, power, andefficiency of the machines. Increasing the size of the stator and rotorback iron can avoid this MMF drop, but unfortunately, it increases themotor size, weight, and cost. Using bipolar excitation of phases canshorten the flux path, but they need a complex converter. Also, they arenot applicable when there is no overlapping in conduction of phases.

FIG. 1A shows a schematic representation of a conventional switchedreluctance motor (SRM) 100. The SRM 100 of FIG. 1A includes a stator 110and a rotor 140. The stator 110 includes eight stationary stator poles120 (each with its own inductor coil 130) and the inner rotor 140includes six rotating rotor poles 150 (no coils). The components of theSRM 100 are typically constructed from magnetic materials such as iron,nickel, or cobalt. In particular configurations, the materials of theSRM 100 can be laminated to reduce the effect of eddy currents. At anyone time, a pair of opposing coils 130 is energized electrically. Theinner magnetic material in the rotor poles 150 of the rotor 140 areattracted to the energized coil 130 causing the entire inner rotor 140to rotate while producing torque. Once alignment is achieved, the pairof opposing coils 130 is de-energized and the next pair of opposingcoils 130 is energized. This sequential firing of coils 130 causes therotor 140 to rotate while producing torque. An illustration is providedwith reference to FIG. 1B.

FIG. 1B is a dot representation of the SRM 100 of FIG. 1A. The whitecircles represent the stator poles 120 and the black circles representthe rotor poles 150. Stator poles 120A, 120B are currently aligned withrotor poles 150A, 150B. Accordingly, the coils associated with thisalignment (coils associated with stator poles 120A, 120B) can bede-energized and another set of coils can be fired. For example, if thecoils associated with the stator poles 120C and 120D are fired, rotorpoles 150C, 150D will be attracted, rotating the rotor 140counter-clockwise. The SRM 100 of FIG. 1 has inherent two-fold symmetry.

FIG. 2 shows a schematic representation of a long flux path through theconventional switched reluctance motor (SRM) 100 of FIG. 1A. In the SRM100, magnetic fluxes must traverse 180 degree through both the stator110 and the rotor 140—for example, through stator pole 120G, rotor pole150G, rotor pole 150H, stator pole 120H, and inner rotor 140, itself.Such long flux paths can lead to the creation of undesirably eddies,which dissipate energy as heat. Additionally, due to the high fluxdensity, the magneto motive force (MMF) drop will be very high,particularly if the stator 110 and rotor 140 back iron are thin.

As an example of MMF drop, FIG. 3 shows in a chart 105 the effect of MMFdrop in the torque production of a one-phase, one horsepower machine. InFIG. 3, output torque 170 is plotted against rotor angle 160. Line 180show torque without the effect of saturation in the rotor 140 and stator110 back iron and line 190 shows torque with the effect of saturation inrotor 140 and stator 110 back iron. As can be seen, the MMF drop intorque production can be more than 6%. Accordingly, teachings of someembodiments reduce the length of the flux path. Further details of suchembodiments will be described in greater detail below.

FIG. 4 shows a dot representation for a switched reluctance motor (SRM)200, according to an embodiment of the invention. The SRM 200 of FIG. 4may operate in a similar manner to the SRM described with reference toFIG. 1B. However, whereas the SRM 100 of FIG. 1B fire two coilsassociated with two stator pole 120 at a time, the SRM of FIG. 4 firesfour coils associated with four stator poles 220 at a time. Theincreased firing of such coils/stator poles 220 increases the torque.

The SRM 200 of FIG. 4 has a rotor with eight rotor poles 250 and astator with twelve stator poles 220. The active magnetized sets ofstator poles 220 are denoted by arrowed lines 225 and the attractiveforces through the flux linkages (e.g., between a rotor pole 250 andstator pole 220) are shown by the shorter lines 235 through acounterclockwise progression of 40° of rotor rotation. At 45°, theconfiguration would appear identical to the 0° configuration. As can beseen with reference to these various rotor angles, as soon as aalignment between four stator poles 220 and four rotor poles 250 occur,four different stator poles 220 are fired to attract the rotor poles 250to the four different stator poles 220.

The switched reluctance motor 200 in FIG. 4 has four-fold symmetry. Thatis, at any one time, four stator poles 220 (the sets denoted by arrowedlines 225) are energized, which as referenced above, is twice as many asa conventional switched reluctance motor (e.g., SRM 100 of FIG. 1).Because twice as many stator poles 220 are energized, the torque isdoubled.

In particular embodiments, adding more symmetry will further increasetorque. For example, six-fold symmetry would increase the torque bythree times compared to a conventional switched reluctance motor. Inparticular embodiments, increased symmetry may be achieved by making therotor as blade-like projections that rotate within a U-shaped stator,for example, as described below with reference to the embodiments ofFIGS. 5A and 5B. In other embodiments, increased symmetry may beachieved in other manners as described in more details below.

FIGS. 5A and 5B illustrate a rotor/stator configuration 300, accordingto an embodiment of the invention. For purposes of illustration, theembodiment of the rotor/stator configuration 300 of FIGS. 5A and 5B willbe described as a switched reluctance motor (SRM). However, as brieflyreferenced above, in particular embodiments, the rotor/statorconfiguration 300 may be utilized as other types of motors. And, inother embodiments, the rotor/stator configuration 300 may be utilized inother types of electric machines such as generators.

In the rotor/state configuration 300 of FIGS. 5A and 5B, a blade-likerotor pole or blade 350, affixed to a rotating body 340, is shownpassing through a U-shaped electromagnet core or U-shaped stator pole320. In this configuration, the flux path is relatively short, comparedto conventional SRMs. For example, the magnetic flux produced by a coil330 fired on the U-shaped pole 320 would pass through one leg 322 of theU-shaped stator pole 320 through the blade 350 and to the other leg 324of the U-shaped stator pole 320 in a circular-like path. In particularembodiments, this short path—in addition to diminishing the long pathdeficiencies described above—enables increased symmetry because the pathdoes not traverse the center of the rotating body 340 and has littleeffect, if any, on other flux paths. Additionally, in particularembodiments, the short path enables use of the center of the rotatingbody 340 for other purposes. Further details of such embodiments will bedescribed below. Furthermore, radial loads are applied to the rotor withthis embodiment and axial loads on the rotor are balanced. Additionally,extra radius is afforded by the blade 350, thus increasing generatedtorque.

The following is a first order analysis of the electromagneticinteraction between a single blade 350/pole 320 set, according to anembodiment of the invention. The schematic of FIGS. 5A and 5B is a blade350/pole 320 set where r_(i) defines the outer boundary of the outercompressor rotor 340, r_(o), is the radius at the blade tips, Δr is theradial length of the blade 350, α is the angular dimension of the blade350, β is the angular dimension of the coil 330, θ is the angularengagement of the rotor blade 350 within the coil 330, A(θ) is the areaavailable for flux linkage, g is the gap dimension on each side of theblade 350, W_(b) is the width of the blade 350, and w_(c) is the widthof the core.

The magnetic flux through the magnetic circuit created is:

$\begin{matrix}{\phi = \frac{Ni}{\Re_{c} + \Re_{g}}} & {{{Eq}.\mspace{14mu} 1}A}\end{matrix}$where N is the number of turns in the coil 330, i is the current throughthe coil 330, and R_(c) and R_(g) are the reluctances of the core andthe air gap, respectively. The reluctances are

$\begin{matrix}{{\Re_{c} = \frac{l_{c}}{\mu\; A_{c}}}{\Re_{g} = \frac{2g}{\mu_{o}A_{g}}}} & {{{Eq}.\mspace{14mu} 2}A}\end{matrix}$where l_(c) is the flux length of the core material, μ is thepermeability of the core material, A_(c) is the cross sectional area ofthe core, g is the air gap thickness, μ_(o) is the permeability of freespace (in the air gap), and A_(g) is the area of the gap over which fluxlinkage occurs. Two gaps, one on either side of the blade 350, have beenaccounted for in the reluctance expression. The magnetic reluctance, R,is analogous to electrical resistance. Because the permeability of thecore material is far greater than that of air, the reluctance of the airgap dominates in Eq. 1A, so substituting the expression for R_(g) intoEq. 1A gives

$\begin{matrix}{\phi \approx \frac{{Ni}\;\mu_{o}A_{g}}{2g}} & {{{Eq}.\mspace{14mu} 3}A}\end{matrix}$Because the air gap has been assumed to dominate the total reluctance,the inductance, L, can be expressed as

$\begin{matrix}{L = {\frac{\lambda}{i} = {\frac{N\;\phi}{i} = \frac{N^{2}\mu_{o}A_{g}}{2g}}}} & {{{Eq}.\mspace{14mu} 4}A}\end{matrix}$where λ=Nφ is the flux linkage.

The stored energy in the field is given by

$\begin{matrix}{W_{fld} = {\frac{1}{2}{\frac{\lambda^{2}}{L}.}}} & {{{Eq}.\mspace{14mu} 5}A}\end{matrix}$

An expression for L(θ) is required in Eq. 5A. Under present assumptions,the only reason for the inductance to vary with rotor angle is that theflux linkage area over the air gap, A_(g), changes with rotation. FromFIGS. 5A and 5B, the following relationship between θ and A_(g) can bewritten asA _(g)(r*θ)=2Δr(r*θ)  Eq. 6Abecause the air gap dimensions change by sweeping the radial spanΔr=r_(o)−r_(i) over the arc length r*θ, where r*=½(r_(o)+r_(i)). Thus

$\begin{matrix}{{{L\left( {r*\theta} \right)} = \frac{N^{2}\mu_{o}\Delta\;{r\left( {r*\theta} \right)}}{g}},} & {{{Eq}.\mspace{14mu} 7}A}\end{matrix}$and Eq. 5A will be modified to

$\begin{matrix}{W_{fld} = {\frac{1}{2}{\frac{\lambda^{2}}{L\left( {r*\theta} \right)}.}}} & {{{Eq}.\mspace{14mu} 8}A}\end{matrix}$Substituting Eq. 7A into Eq. 8A gives

$\begin{matrix}{W_{fld} = {{\frac{1}{2}\frac{\lambda^{2}g}{N^{2}\mu_{o}\Delta\;{r\left( {r*\theta} \right)}}} = {\frac{\lambda^{2}g}{2N^{2}\mu_{o}\Delta\;{r\left( {r*\theta} \right)}}.}}} & {{{Eq}.\mspace{14mu} 9}A}\end{matrix}$From conservation of energy, dW_(fld) can be expressed asdW _(fld)(λ, r*θ)=idλ−f _(fld) d(r*θ)  Eq. 10AThe total derivative of dW_(fld) with respect to the independentvariables λ and r*θ is

$\begin{matrix}{{dW}_{fld} = {\left( {\lambda,{r*\theta}} \right) = {{\frac{\partial W_{fld}}{\partial\lambda}d\;\lambda} + {\frac{\partial W_{fld}}{\partial\left( {r*\theta} \right)}{{d\left( {r*\theta} \right)}.}}}}} & {{{Eq}.\mspace{14mu} 11}A}\end{matrix}$Observation of Eq. 10A and 11A indicates that

$\begin{matrix}\begin{matrix}{f_{fld} = {- \frac{\partial W_{fld}}{\partial\left( {r*\theta} \right)}}} \\{= {- {\frac{\partial\;}{\partial\left( {r*\theta} \right)}\left\lbrack \frac{\lambda^{2}g}{2N^{2}\mu_{o}\Delta\;{r\left( {r*\theta} \right)}} \right\rbrack}}} \\{= {\frac{\lambda^{2}g}{2N^{2}\mu_{o}\Delta\;{r\left( {r*\theta} \right)}^{2}}.}}\end{matrix} & {{{Eq}.\mspace{14mu} 12}A}\end{matrix}$Substituting λ=L(r*θ)i from Eq. 4A into Eq. 12A, the followingdependence of f_(fld) on coil current, i, is obtained:

$\begin{matrix}{f_{fld} = {\frac{N^{2}µ_{o}\Delta\; r}{2g}{i^{2}.}}} & {{{Eq}.\mspace{14mu} 13}A}\end{matrix}$Ultimately, the torque produced from f_(fld) acting at a radius r* isneeded for an individual blade 350/pole 320 set. This resulting torqueis

$\begin{matrix}{T_{fld} = {\frac{N^{2}µ_{o}\Delta\; r}{2g}r^{*}{i^{2}.}}} & {{{Eq}.\mspace{14mu} 14}A}\end{matrix}$SRM Torque Generation

The key result of the above analysis is the following equation for thetorque generated by a single blade 350/pole 320 set interaction asdepicted in FIGS. 5A and 5B:

$\begin{matrix}{T_{fld} = {\frac{N^{2}µ_{o}\Delta\; r}{2g}r^{*}i^{2}}} & (1)\end{matrix}$In Eq. 1, T_(fld) is the torque generated by the magnetic field, N isthe total number of winding encirclements around the stator pole core,μ_(o) is the permeability of free space, Δr=r_(o)−r_(i) (radialdimension of the rotor blade), r*=r_(i)+Δr/2(radius to the bladecenter), i is the coil current, and g is the air gap dimension.

In particular embodiments, a rotor/stator configuration (e.g., therotor/stator configuration 300 of FIGS. 5A and 5B) can be integratedwith other features such as a gerotor compressor and other embodimentsdescribed in the following United States Patents and Patent ApplicationPublications, the entirety of which are hereby incorporated byreference: Publication No. 2003/0228237; Publication No. 2003/0215345;Publication No. 2003/0106301; U.S. Pat. Nos. 6,336,317; and 6,530,211.

The following assumptions may be made with the application of Eq. 1 todesign an integral compressor/SRM:

-   -   1) laminated Sofcomag (2.3 Tesla saturation limit) is used to        carry magnetic flux    -   2) magnetic flux is limited to 2.0 Tesla, below saturation    -   3) four poles are magnetized at any given time    -   4) fringe effects in the laminates are ignored        As an example, an industrial compressor requires roughly 2.6 MW.        Operating at 3,600 rpm, the torque required is 6,896 N-m.        Appropriate selection and sizing of the rotor to process the        specified capacity yields r_(i)=14 in (0.3556 m). A reasonable        gap dimension given thermal expansion and bearing play is        g=0.080 in (0.00203 m). With assumption 2, the maximum        ampere-turn product may be calculated such that a 2 Tesla flux        density is not exceeded. Also from the above analysis,

$\begin{matrix}{{Ni} = {\frac{2{gB}}{µ_{o}}.}} & (2)\end{matrix}$The maximum product of Ni can be calculated as 6,468 A. Becauser*=r_(i)+Δr/2, Δr is selected along with the number of blade/pole arraysstacked in the axial direction to satisfy the torque requirement.Recalling that four blade/pole sets are active at a given instant intime and letting m be the number of stacked arrays, the total torque isT_(tot)=4T_(fld)m  (3)For Δr=4.5 in (0.127 m), r*=16.5 in (0.4191 m). Letting m=3, T_(tot) canbe calculated as 7,323 N-m. The resulting power output at 3,600 rpm is2.76 MW.Design Case Implementation

FIGS. 6-10 illustrate a rotor/stator configuration 450, according to anembodiment of the invention. The rotor/stator configuration 450 of FIGS.6-10 is used with a compressor. However, as briefly referenced above, inparticular embodiments, the rotor/stator configuration 450 may beutilized as other types of motors and other types of electric machinessuch as generators. The rotor/stator configuration 450 of FIGS. 6-10includes three stacked arrays of twelve stator poles 444 and eight rotorblades 412. The rotor/stator configuration 450 for the compressor inFIGS. 6-10 may operate in a similar manner to the rotor/statorconfiguration 300 described above with reference to FIGS. 5A and 5B.FIG. 6 shows an outer rotor assembly 400 of the rotor/statorconfiguration 450, according to an embodiment of the invention. Theouter rotor assembly 400 in FIG. 6 includes a bearing cap 402, a bearingsleeve 404, a port plate 406, inlet/outlet ports 408, two rotor segments410A/410B with rotor blades 412 mounted, a seal plate 414 to separatethe dry compression region from the lubricated gear cavity, arepresentation of the outer gear 416 (internal gear), an end plate 418with blades 412 mounted, an outer rear bearing 420, and another bearingcap 422. In this embodiment, the outer compressor rotor serves as therotor for the SRM.

In this embodiment, there are eight outer rotor lobes 411 with eightblades 412 in each radial array 413 of rotor poles. In particularembodiments, such symmetry may be necessary to minimize centrifugalstress/deformation. In this configuration, ferromagnetic materialsutilized for the operation of the rotor/stator configuration 450 mayonly be placed in the blades 412 of the radial array 413.

FIG. 7 shows an inner rotor assembly 430 of the rotor/statorconfiguration 450, according to an embodiment of the invention. Theinner rotor assembly 430 of FIG. 7 includes an inner shaft 432, a stackof three (seven lobed) inner rotors 434A/434B/434C, a spur gear 436, andan inner rear bearing 438.

Details of operation of the inner rotor assembly 430 with respect to theouter rotor assembly 400, according to certain embodiments of theinvention, as well as with other configuration variations are describedin further detail in one ore more of the following United States Patentsand/or Patent Application Publications, which as referenced above areincorporated by reference: Publication No. 2003/0228237; Publication No.2003/0215345; Publication No. 2003/0106301; U.S. Pat. Nos. 6,336,317;and 6,530,211.

FIG. 8 shows a stator/compressor case 440 of the rotor/statorconfiguration 450, according to an embodiment of the invention. Thestator/compressor case 440 of FIG. 8 in this embodiment includes threestacks 442A, 442B, 442C of twelve stator poles 444, spaced at equalangles. Although the stator poles 444 could be mounted to the case 440in many ways, an external coil embodiment is shown in FIG. 8. There aretwo coils 446A, 446B per stator pole 444, which are mounted in sets ofthree into a nonferromagnetic base plate 448, forming a bolt-in polecartridge 450. In particular embodiments, the coils 446A, 446B may becopper coils. In other embodiments, the coils 446A, 446B may be made ofother materials. In particular embodiments, the number of coils 446 on agiven stator pole 444 can be increased above two, thereby reducing thevoltage that must be supplied to each coil. During operation ofparticular embodiments, all poles in four cartridges 450 (90° apart) maybe magnetized simultaneously. The magnetization occurs sequentiallycausing the outer rotor assembly 400 of FIG. 6 to rotate.

FIG. 9 shows a cutaway view of a composite assembly 460 of arotor/stator configuration 450, according to an embodiment of theinvention. The composite assembly 460 shows an integration of the outerassembly 400, the inner assembly 430, and the stator/compressor case 440of FIGS. 6-8 as well as end plates 462 providing bearing support and gasinlet/outlet porting through openings 464. FIG. 10 shows the compositeassembly 460 without the cutaway.

In certain embodiments, during operation, the rotor may expand due tocentrifugal and thermal effects. To prevent contact between the rotorpoles and stator poles, a large air gap is typically used. Equation 1above described with reference to FIGS. 5A and 5B shows that the torqueis strongly affected by the air gap. A smaller gap results in moretorque. Accordingly, there are advantages to reducing the gap as smallas possible. Teachings of some embodiments recognize configurations formaintaining small gap during thermal and centrifugal expansion of arotor.

FIG. 11 shows a side view of how a rotor 540 changes shape when itexpands due to centrifugal and thermal effects. The rotor 540 has anaxis of rotation 503. The solid line 505 represents the rotor 540 priorto expansion and the dotted line 507 represents the rotor 540 afterexpansion. Dots 510A, 512A, and 514A represent points on the rotor 540at the cold/stopped position and dots 510C, 512C, and 514C represent thesame points on the rotor 540 at the hot/spinning position. The left edgeor thermal datum 530 does not change because it is held in place whereasthe right edge is free to expand. The trajectories 510B, 512B, and 514Bof dots is purely radial at the thermal datum 530 and becomes more axialat distances farther from the thermal datum 530.

FIG. 12 shows a rotor/stator configuration 600, according to anembodiment of the invention. The rotor/stator configuration 600 includesa rotor 640 that rotates about an axis 603. The rotor 640 includes rotorpoles 650 that interact with stator poles 620, for example, upon firingof coils 630. The rotor/stator configuration 600 of FIG. 12 may operatein a similar manner to the rotor/stator configuration 300 of FIGS. 5Aand 5B, except for an interface 645 between the rotor pole 650 and thestator pole 620. In the rotor/stator configuration 600 of FIG. 12, anangle of interface 645 between the rotor pole 650 and stator pole 620 isthe same as the trajectory of a dot on the surface of the rotor 540shown in FIG. 11. By matching these angles, the surface of the rotorpole 650 and the surface of the stator pole 620 slide past each otherwithout changing an air gap 647, even as the rotor 640 spins and heatsup. This design allows for very small air gaps to be maintained even ata wide variety of rotor temperatures. In particular embodiments, thehousing that holds the stator pole 620 may be assumed to be maintainedat a constant temperature. Various different angles of interface 645 maybe provided in a single configuration for a rotor pole 650/stator pole620 pair, dependant upon the trajectory of the dot on the surface of therotor 640.

FIGS. 13A and 13B show a rotor/stator configuration 700A, 700B,according to another embodiment of the invention. The rotor/statorconfigurations 700A, 700B include rotors 740 that rotate about an axis703. The rotor/stator configurations 700A, 700B of FIGS. 13A and 13B mayoperate in a similar manner to the rotor/stator configuration 300 ofFIGS. 5A and 5B, including rotor poles 750, stator poles 720A, 720B, andcoils 730A, 730B. The rotor/stator configuration 700A of FIG. 13A showthree U-shaped stators 720A, operating as independent units. Therotor/stator configuration 700B of and FIG. 13B shows a single E-shapedstators 710B operating like three integrated U-shaped stators 720A. ThisE-shaped stator 720B allows for higher torque density. Although anE-shaped stator 720B is shown in FIG. 13B, other shapes may be used inother embodiments in integrating stator poles into a single unit.

FIG. 14 shows a rotor/stator configuration 800, according to anotherembodiment of the invention. In a similar manner to that described abovewith other embodiments, the rotor/stator configuration 800 of FIG. 14may be utilized with various types of electric machines, includingmotors and generators. The rotor/stator configuration 800 of FIG. 14 mayoperate in a similar manner to the rotor/stator configuration 300 ofFIGS. 5A and 5B, including rotor poles 850 and U-shaped stator poles820. However, the stator poles 820 have been axially rotated ninetydegrees such that the rotor poles 850 do not transverse between a gap ofthe U-shape stator poles 820. Similar to FIGS. 5A and 5B, the flux pathis relatively short. For example, the magnetic flux produced by a coilfired on the U-shaped pole 820 would pass through one leg 822 of thepole 820 through the rotor pole 850 through a periphery of the rotorthrough another rotor pole 850 and to the other leg 824 of the pole 820in a circular-like path.

The rotor/stator configuration 800 of FIG. 14 is shown with three phasesA, B, and C and two pairs of stator poles 820 per each phase. In thisembodiment, stator poles 820 are U-shaped iron cores with coils that areinserted into a non-ferromagnetic yoke 890. In other embodiments thestator poles 820 may be made of materials other than iron and may haveother configurations. The stator poles 820 in particular embodiments maybe electrically and magnetically isolated from each other. The rotor 840in the embodiment of FIG. 14 may operate like a rotor of a conventionalSRM; however, unlike a conventional SRM, the pitches of the rotor pole850 and stator pole 820 are the same.

The magnetic reluctance of each phase changes with position of the rotor840. As shown in FIG. 15, when a rotor pole 850 is not aligned with twostator poles 820, the phase inductance is at a minimum and this positionmay be called an unaligned position. When the rotor pole 850 is alignedwith the stator pole 820, the magnetic inductance is at a maximum andthis position may be called an aligned position. Intermediate betweenthe aligned position and unaligned position is an intermediate position.SRM torque is developed by the tendency of the magnetic circuit to findthe minimum reluctance (maximum inductance) configuration.

The configuration of FIG. 14 is such that whenever the rotor 840 isaligned with one phase, the other two phases are half-way aligned, sothe rotor 840 can move in either direction depending which phase will beexcited next.

For a phase coil with current i linking flux, the co-energy W′ can befound from the definite integral:

$\begin{matrix}{W^{\prime} = {\int_{0}^{i}{\lambda{\mathbb{d}i}}}} & (4)\end{matrix}$The torque produced by one phase coil at any rotor position is given by:

$\begin{matrix}{T = \left\lbrack \frac{\partial W^{\prime}}{\partial\theta} \right\rbrack_{i = {constamt}}} & (5)\end{matrix}$The output torque of an SRM is the summation of torque of all phases:

$\begin{matrix}{T_{m} = {\sum\limits_{j = 1}^{N}{T\left( {i_{j},\theta} \right)}}} & (6)\end{matrix}$If the saturation effect is neglected, the instantaneous torque can begiven as:

$\begin{matrix}{T = {\frac{1}{2}i^{2}\frac{\mathbb{d}L}{\mathbb{d}\theta}}} & (7)\end{matrix}$

From Equation 7, it can be seen that to produce positive torque(motoring torque) in SRM, the phase has to be excited when the phasebulk inductance increases, which is the time that the rotor movestowards the stator pole. Then it should be unexcited when it is inaligned position. This cycle can be shown as a loop in flux linkage(λ)—phase current (i_(ph)) plane, which is called energy conversion loopas shown in FIG. 16. The area inside the loop (S) is equal to theconverted energy in one stroke. So the average power (P_(ave)) and theaverage torque of the machine (T_(ave)) can be calculated as follows:

$\begin{matrix}{P_{ave} = \frac{N_{p}N_{r}N_{s}S\;\omega}{4\pi}} & (8) \\{T_{ave} = \frac{N_{p}N_{r}N_{ph}S}{4\pi}} & (9)\end{matrix}$where, N_(p), N_(r), N_(ph), ω are the number of stator pole pairs perphase, number of rotor poles, number of stator phases, and rotor speed,respectively.

By changing the number of phases, stator pole pitch, and statorphase-to-phase distance angle, different types of short-flux-path SRMscan be designed.

FIG. 17 shows a rotor/stator configuration 900, according to anotherembodiment of the invention. The rotor/stator configuration 900 of FIG.17 is a two-phase model, which operates in a similar manner to the modeldescribed with reference to FIG. 14. The configuration 900 of FIG. 17includes rotor 940; rotor poles 950; stator poles 920; legs 922, 924;and yoke 990.

FIG. 18 shows a rotor/stator configuration 1000, according to anotherembodiment of the invention. In a similar manner to that described abovewith other embodiments, the rotor/stator configuration 1000 of FIG. 18may be utilized with various types of electric machines, includingmotors and generators. The rotor/stator configuration 1000 of FIG. 18may operate in a similar manner to rotor/stator configuration 1000 ofFIG. 14, including U-shaped stator poles 1020, rotor poles 1050, anon-ferromagnetic yoke 1080, and phases A, B, and C. However, in therotor/stator configuration 1000 of FIG. 18, the rotor poles 1050 areplaced radially outward from the stator poles 1020. Accordingly, therotor 1040 rotates about the stator poles 1020. Similar to FIG. 14, theflux path is relatively short. For example, the magnetic flux producedby a coil fired on the U-shaped pole 1020 would pass through one leg1022 of the stator pole 1020 through the rotor pole 1050 and to theother leg 1024 of the stator pole 820 in a circular-like path. As oneexample application of the rotor/stator configuration 1000 according toa particular embodiment, the rotor/stator configuration 1000 may be amotor in the hub of hybrid or electric (fuel cell) vehicles, and others.In this embodiment, the wheel is the associated with the rotor 1040,rotating about the stators 1020. This rotor/stator configuration 1000may additionally be applied to permanent magnet motors, for example, asshown in FIG. 19.

FIG. 19 shows a rotor configuration 1100, according to anotherembodiment of the invention. The rotor/stator configuration 1100 of FIG.14 may operate in a similar manner to rotor/stator configuration 1100 ofFIG. 14, including U-shaped stator poles 1120, a non-ferromagnetic yoke1190, and phases A, B, and C, except that a rotor 1140 containsalternating permanent magnet poles 1152, 1154.

FIG. 20 shows a rotor/stator configuration 1200, according to anotherembodiment of the invention. In a similar manner to that described abovewith other embodiments, the rotor/stator configuration 1200 of FIG. 20may be utilized with various types of electric machines, includingmotors and generators. The rotor/stator configuration 1200 of FIG. 20integrates several concepts described with reference to otherembodiments, including blades 1250A, 1250B from FIGS. 5A and 5B;E-shaped stator poles 1220A, 1220B from FIG. 13B; stator poles 1220Bradially inward of rotor poles 1250B from FIGS. 6-10; and stator poles1220A radially outward of rotor poles 1250B from FIG. 18. The statorpoles 1220A are rigidly mounted both on the inside and outside of a drum1285, which allows torque to be applied from both the inside and outsidethereby increasing the total torque and power density. In particularembodiments, the rotor poles 1250A, 1250B may be made of a ferromagneticmaterial, such as iron, which is a component of a switched reluctancemotor. In other embodiments, the rotor poles 1250A, 1250B could bepermanent magnets with the poles parallel to the axis of rotation, whichwould be a component of a permanent magnet motor.

FIGS. 21A and 21B show a rotor/stator configuration 1300, according toanother embodiment of the invention. In a similar manner to thatdescribed above with other embodiments, the rotor/stator configuration1200 of FIGS. 21A and 21B may be utilized with various types of electricmachines, including motors and generators. The rotor/statorconfiguration 1300 of FIGS. 21A and 21B may operate in a similar mannerto the rotor/stator configuration 1300 of FIGS. 5A and 5B, includingrotor poles 1350 and U-shaped stator poles 1320. However, the rotorpoles 1350 and U-shaped stator poles 1320 have been rotated ninetydegrees such that rotor poles 1350 rotate between a leg 1322 of thestator pole 1320 that is radially inward of the rotor pole 1350 and aleg 1324 of the stator pole 1320 that is radially outward of the rotorpole 1350. In the embodiment of the rotor/stator configuration 1300 ofFIGS. 21A and 21B, it can be seen that the axial and radial fluxesco-exist.

In this embodiment and other embodiments, there may be no need for amagnetic back-iron in the stator. Further, in this embodiment and otherembodiments, the rotor may not carry any magnetic source. Yet further,in particular embodiments, the back iron of the rotor may not need to bemade of ferromagnetic material, thereby creating flexibility design ofthe interface to the mechanical load.

In this embodiment and other embodiments, configuration may offer higherlevels of power density, a better participation of stator and the rotorin force generation process and lower iron losses, thereby offering agood solution for high frequency applications. In various embodimentsdescribed herein, the number of stator and rotor poles can be selectedto tailor a desired torque versus speed characteristics. In particularembodiments, cooling of the stator may be very easy. Further, themodular structure of certain embodiments may offer a survivableperformance in the event of failure in one or more phases.

Optimization of the Magnetic Forces

FIGS. 22-25 illustrate an optimization of magnetic forces, according toembodiments of the invention. The electromagnetic force on the surfaceof a rotor has two components, one that is perpendicular to thedirection of motion and one that is tangent to the direction of motion.These components of the force may be referred to as normal andtangential components of the force and can be computed from magneticfield quantities according to the following equations:

$f_{n} = {\frac{1}{2\mu_{0}}\left( {B_{n}^{2} - B_{t}^{2}} \right)}$$f_{t} = {\frac{1}{\mu_{0}}B_{n}B_{t}}$For an optimal operation, the tangential component of the force needs tobe optimized while the normal component of the force has to be kept at aminimal level or possibly eliminated. This, however, is not the case inconventional electromechanical converters. To the contrary, the normalforce forms the dominant product of the electromechanical energyconversion process. The main reason for this can be explained by thecontinuity theorem given below. As the flux lines enter from air into aferromagnetic material with high relative permeability the tangentialand normal components of the flux density will vary according to thefollowing equations:

B_(n, air) = B_(n, iron)$B_{t,{air}} = {\frac{1}{\mu_{r,{iron}}}B_{t,{iron}}}$The above equations suggest that the flux lines in the airgap will enterthe iron almost perpendicularly and then immediately change directiononce enter the iron. This in turn suggests that in a SRM and on thesurface of the rotor we only have radial forces.

FIG. 22 illustrates the formation of flux lines in a SRM drive. The fluxdensity, B, is shown in Teslas (T). The radial forces acting on theright side of the rotor (also referred to as fringing flux—indicated byarrow 1400) create radial forces (relative to the rotor surface) thatcreate positive propelling force for the rotor. This is the area thatneeds attention. The more fluxes are pushed to this corner, the bettermachine operates. This explains why SRM operates more efficient undersaturated condition. This is because due to saturation, the effectiveairgap of the machine has increased and more flux lines are choosing thefringing path.

To enhance the migration of flux lines towards the fringing area, oneembodiment of the invention uses a composite rotor surface. In thecomposite rotor surface, the top most part of the of the rotor is formedby a material that goes to saturation easier and at a lower fluxdensity, thereby reinforcing the fringing at an earlier stage of theelectromechanical energy conversion process. In particular embodiments,the shape of the flux barrier or the shape of the composite can beoptimized to take full advantage of the magnetic configuration. Inanother embodiment, flux barriers can be introduced in the rotor todiscriminate against radial fluxes entering the rotor normally and pushmore flux lines towards the fringing area. FIGS. 23, 24 and 25illustrate these embodiments.

FIGS. 23 and 24 show the placement of easily saturated materials or fluxbarriers 1590A, 1590B, 1590C, and 1590D under the surface of rotors1550A, 1550B, and stators 1520A, 1520B. Example materials for easilysaturated materials or flux barriers 1590 include, but are not limitedto M-45. Example ferromagnetic materials for the rotors 1550 and stators1520 include, but are not limited HyperCo-50. The shape, configuration,and placement of the easily saturated materials or flux barriers maychange based on the particular configurations of the rotors and stators.

FIG. 25 shows a chart 1600 of B-H curve for various alloys. The chart1600 of FIG. 25 charts magnetic flux density 1675, B, against magneticfield 1685, H, for alloys 1605, 1615, and 1625.

The short-flux-path configurations described with reference to severalembodiments herein may be implemented for any SRM application, bychanging the number of stator and rotor poles and sizes. Similarconfiguration may be utilized for axial-field and linear motors.

Several embodiments described herein may additionally be used forpermanent magnet AC machines where the rotor contains alternatingpermanent magnet poles. Both of these families of machines, SRM andBLDC, may be used as both motors and generators.

Additionally, the embodiments described above may be turned inside outand used as an interior stator SRM or BLDC machine, with the rotor onthe outside. These in turn can be used both for motoring or generatingor both.

Numerous other changes, substitutions, variations, alterations, andmodifications may be ascertained to one skilled in the art and it isintended that the present invention encompass all such changes,substitutions, variations, alterations, and modifications as fallingwithin the scope of the appended claims.

1. An electric machine, comprising: a stator having at least one statorpole, the at least one stator pole including a first leg and a secondleg; a rotor having at least one rotor pole, wherein the rotor rotatesrelative to the stator, and the at least one rotor pole is configured torotate between the first leg and the second leg of the at least onestator pole; a first coil disposed on the first leg such that most, ifnot all, of the first coil disposed on the first leg is spaced apartfrom a rotational axis of the rotor by a distance greater than or equalto a maximum radius of rotation of the at least one rotor pole; andwherein the at least one rotor pole and the at least one stator pole areconfigured such that a magnetic flux induced on the at least one rotorpole flows in a direction substantially parallel to a rotational axis ofthe rotor.
 2. The electric machine of claim 1, wherein the at least onestator pole is U-shaped.
 3. The electric machine of claim 1, wherein theat least one rotor pole is a ferromagnetic blade.
 4. The electricmachine of claim 1, wherein the electric machine is selected from thegroup consisting of: a switched reluctance motor; a motor other than aswitched reluctance motor; and a generator.
 5. The electric machine ofclaim 1, wherein the at least one rotor pole comprises at least twopermanent magnets aligned, with respect to each other, parallel to therotational axis of the rotor.
 6. The electric machine of claim 1,wherein the at least one rotor pole and the at least one stator pole areconfigured such that a magnetic flux induced on the at least one rotorpole flows from the first leg through the at least one rotor pole to thesecond leg upon rotation of the at least one rotor pole between thefirst leg and the second leg.
 7. The electric machine of claim 6,wherein the at least one stator pole and the at least one rotor pole areconfigured such that the magnetic flux does not traverse an interior ofthe rotor.
 8. The electric machine of claim 7, wherein an interior ofthe rotor is a compressor.
 9. The electric machine of claim 7, whereinan interior of the rotor is an expander.
 10. The electric machine ofclaim 6, further comprising: at least one coil disposed on the at leastone stator pole, the at least one coil operable to induce the magneticflux.
 11. The electric machine of claim 10, further comprising: a casingseparating an interior portion of the electrical machine from anexterior portion of the electrical machine, wherein the at least onecoil is disposed on an exterior of the casing.
 12. The electric machineof claim 6, further comprising: a second coil disposed on second leg,the first and second coils operable to induce the magnetic flux.
 13. Theelectric machine of claim 1, wherein the stator has a plurality ofstator poles, each of the plurality of stator poles including a firstleg and a second leg, the rotor has a plurality of rotor poles, and eachof the plurality of rotor poles rotate between each of the first legsand the second legs of the plurality of stator poles.
 14. The electricmachine of claim 13, wherein wherein the plurality of stator poles andthe plurality of rotor poles are configured such that a magnetic fluxinduced on each of the plurality of stator poles does not traverse aninterior of the rotor.
 15. The electric machine of claim 14, wherein aninterior of the rotor is a compressor.
 16. The electric machine of claim14, wherein an interior of the rotor is an expander.
 17. The electricmachine of claim 13, further comprising: at least one coil disposed oneach of the plurality of stator poles, the at least one coil operable toselectively induce a magnetic flux.
 18. The electric machine of claim17, further comprising: a casing separating an interior portion of theelectrical machine from an exterior portion of the electrical machine,wherein the at least one coil on each of the plurality of stator polesis disposed on an exterior of the casing.
 19. The electric machine ofclaim 18, wherein each of the plurality of stator poles is disposed in acartridge removably coupleable to the casing.
 20. The electric machineof claim 13, wherein each of the plurality of stator poles iselectrically and magnetically isolated from the rest of the plurality ofstator poles.
 21. The electric machine of claim 20, wherein each of theplurality of rotor poles is a ferromagnetic blade.
 22. The electricmachine of claim 1, wherein the at least one stator pole is more thanfour stator poles, the at least one rotor pole is more than four rotorpoles, and the more than four stator poles and the more than four rotorpoles are configured such that two sets of the more than four statorpoles may be electrically fired at the same time to attract two sets ofthe more than four rotor poles.
 23. An electric machine, comprising: astator having more than four stator poles, each stator pole comprising acoil; and a rotor having more than four rotor poles, wherein the rotorrotates relative to the stator such that each of the rotor polestraverses a radius of rotation disposed radially inward from most, ifnot all, of each coil of each of the stator poles; and the more thanfour stator poles and the more than four rotor poles are configured suchthat two sets of the more than four stator poles may be electricallyfired at the same time to attract two sets of the more than four rotorpole; and wherein the more than four rotor poles and the more than fourstator poles are configured such that a magnetic flux induced on atleast one of the more than four rotor poles flows in a directionsubstantially parallel to a rotational axis of the rotor.
 24. Theelectric machine of claim 23, wherein each set of the two sets of themore than four stator poles are one hundred and eighty degrees part. 25.The electric machine of claim 23, wherein the more than four statorpoles is more than six stator poles, the more than four rotor poles ismore than six rotor poles, and the more than six stator poles and themore than six rotor poles are configured such that three sets of themore than six stator poles may be fired at the same time to attractthree sets of the more than six rotor poles.
 26. The electric machine ofclaim 23, wherein the more than four stator poles are U-shaped.
 27. Theelectric machine of claim 23, wherein the more than four rotor poles areferromagnetic blades.
 28. The electric machine of claim 23, wherein theelectric machine is a motor.
 29. The electric machine of claim 28,wherein the electric machine is a switched reluctance motor.
 30. Theelectric machine of claim 23, wherein each of the more than four statorpoles include a first leg and a second leg, each of the more than fourrotor poles rotate between each of the first legs and the second legs ofthe more than four stator poles.
 31. The electric machine of claim 23,wherein the more than four stator poles and the more than four rotorpoles are configured such that a magnetic flux induced on each of themore than four stator pole does not traverse an interior of the rotor.32. The electric machine of claim 31, wherein an interior of the rotoris a compressor.
 33. The electric machine of claim 31, wherein aninterior of the rotor is an expander.
 34. The electric machine of claim23, wherein each coil is operable to selectively induce a magnetic flux.35. The electric machine of claim 34, further comprising: a casingseparating an interior portion of the electrical machine from anexterior portion of the electrical machine, wherein the at least onecoil on each of the more than four stator poles is disposed on anexterior of the casing.
 36. The electric machine of claim 35, whereineach of the more than four stator poles is disposed in a cartridgeremovably coupleable to the casing.
 37. The electric machine of claim23, wherein each of the more than four stator pole is electrically andmagnetically isolated from the rest of the more than four stator pole.38. The electric machine of claim 23, wherein the more than four rotorpoles each comprise at least two permanent magnets aligned, with respectto each other, parallel to the rotational axis of the rotor.
 39. Anelectric machine, comprising: a stator having at least one stator pole;a rotor having at least one rotor pole, wherein the rotor rotatesrelative to the stator; and the rotor pole comprises at least three fluxbarriers, each flux barrier spaced apart from each other flux barrier,the at least three flux barriers configured to: discriminate againstradial fluxes entering the rotor normally; and push more flux linestoward a fringing area of the rotor.
 40. The electric machine of claim39, further comprising a coil disposed on the at least one stator polesuch that most, if not all, of the coil disposed on the at least onestator pole is spaced apart from a rotational axis of the rotor by adistance greater than or equal to a maximum radius of rotation of the atleast one rotor pole; and wherein the at least one rotor pole and the atleast one stator pole are configured such that a magnetic flux inducedon the at least one rotor pole flows in a direction substantiallyparallel to a rotational axis of the rotor.
 41. The electric machine ofclaim 39, wherein at least a portion of the at least one rotor pole is aferromagnetic blade.
 42. The electric machine of claim 39, wherein theelectric machine is selected from the group consisting of: a switchedreluctance motor; a motor other than a switched reluctance motor; and agenerator.
 43. The electric machine of claim 39, wherein at least one ofthe at least three flux barriers is an air channel.
 44. The electricmachine of claim 39, wherein at least one of the at least three fluxbarriers is an M-45 segment.
 45. The electric machine of claim 39,wherein the at least one stator pole, the at least a first rotor poleand the at least a second rotor pole are configured such that a magneticflux does not traverse an interior of the rotor.
 46. The electricmachine of claim 45, wherein an interior of the rotor is a compressor.47. The electric machine of claim 45, wherein an interior of the rotoris an expander.
 48. The electric machine of claim 45, furthercomprising: at least one coil disposed on the at least one stator pole,the at least one coil operable to induce the magnetic flux.
 49. Theelectric machine of claim 48, further comprising: a casing separating aninterior portion of the electrical machine from an exterior portion ofthe electrical machine, wherein the at least one coil is disposed on anexterior of the casing.